Note that this just puts multiple players into the same game; it doesn't adjust difficulty or ammo pickups or anything else. If you want fancier co-op behaviors then you'll have to use a mod.

2) Less pixellated?

Yes. If you use GLQuake, or any modern Quake engine, then by default there is some filtering of the textures. You can also choose different texture-filtering modes using console commands. (Some modern Quake engines expose this choice in the UI menus.)

Myself though, I like the pixellated look. With the original Quake pixel art, filtering just kinda makes it look blurry IMO. Although I did think it was pretty neat back in the day. (My monitor resolution was a lot smaller then!)

Also yes. :-) I think you can play co-op on any of the maps in the original singleplayer campaign, and on most other singleplayer maps that you can find. Quake does not have any special content reserved for co-op. I think the only thing that makes a map "co-op capable" is that it has multiple player-start positions on it, so that players don't squash each other when the map begins.

Yeah I use Ryger's texture map pack, Darkplaces, and Reforged Bestiary pack. It's awesome I love it. It makes the game look scarier, more atmospheric, and brooding and dark than the original look IMO. I love this game.

The effect of that is not permanent across game restarts, so (slightly longer answer) you will want to make a file named "autoexec.cfg" in your id1 folder that contains +mlook in it. That way it will be applied every time you start the game.

Darkplaces already has mouse support ready to go when you download it then extract it to your quake dir, it gives you the filtering options you want plus more. Basically it has all the stuff you need to play and control your game like a modern shooter. I have never had to mess with console commands or ini files etc. Never needed to really. But yeah, Darkplaces is a solid source port imo one of the best functioning out there. I find it superiod over winquake and GLquake. Also you don't need ryger's text pack to run it. Your game will look like the orginal, instead of what my game looks like. Try it. Google Darkplaces for Quake.

Uuuuhm but darkplace is a graphic mod, I still want to play with GLQuake graphics.

It's not a mod it's a open source port like WInquake and GLquake only better imo. You can play with default graphics with darkplaces, I thought I made that very clear? Anyway, the mods are optional DP plays the game as is.

You get higher resoutions on GLquake, but they aren't in widescreen format such as 16:9 ratio or 16:10. At least the last time I checked anyway, I dunno I could be wrong now. Anyway it's that reason why I use Darkplaces it has 16:9 support and full widescreen. Peace.

FWIW, any of the OpenGL Quake engines (old or new) can use any resolution that your monitor supports. Although for GLQuake you do need version 0.98 if you want to be able to fix your statusbar size/stretch and your FOV.